Stanford researchers have found that concentric carbon nanotubes, with the outer layer riddled by defects and impurities, could be a cheap alternative for some of the platinum catalysts that convert hydrogen and oxygen into water in fuel cells and metal-air batteries.

Nanotubes are made of graphene, a one-atom-thick rolled-up sheet of carbon. Abundant structural defects on the outside of the nanotube increase the kind of catalytic reactions that are vital to batteries and fuel cells. But if the inside of the nanotube is damaged, its conductivity drops uncontrollably, degrading performance. Damaging the outside of a frail one-atom-thick structure while leaving the inside intact, however, appeared to be a nearly impossible feat.

The Stanford team solved this dilemma by building structures made of two to three concentric nanotubes. Treating the multi-walled nanotubes in a chemical solution only damaged the outer tube, increasing the number of defects and impurities on the outside but leaving the inside intact.

Originally, the team planned to test the effect of nitrogen impurities alone. But by a pure stroke of luck, traces of iron, the growth seeds used to manufacture the carbon nanotubes, were added into the mix. When the impurities (0.24 percent iron and 5.3 percent nitrogen) were chemically cleared out, the overall performance dropped, suggesting that the impurities do indeed play a vital role in the structure.

In fact, the researchers found that the performance of their nanotubes is very close to platinum, and say that the high stability of the design makes them the perfect candidate for fuel cells and metal-air batteries- lithium-air in particular.

"Lithium-air batteries are exciting because of their ultra-high theoretical energy density, which is more than 10 times higher than today's best lithium ion technology," said Prof. Hongjie Dai, co-author of the study. "One of the stumbling blocks to development has been the lack of a high-performance, low-cost catalyst. Carbon nanotubes could be an excellent alternative to the platinum, palladium and other precious-metal catalysts now in use."

This drawing shows a double-walled carbon nanotube. Each tube is made of a rolled-up sheet of carbon that’s one-atom thick (Image: Guosong Hong/Stanford University)

This microscopy image shows the damaged outer walls of double- and triple-walled carbon nanotubes (CNT) after being treated in a chemical solution. This technique could help make carbon nanotubes an attractive, low-cost alternative to platinum catalysts in fuel cells (Image: Hongjie Dai Lab/Stanford University)

The damaged outer wall of a carbon nanotube with nanosized graphene pieces (white patches), which facilitate the formation of catalytic sites made of iron (yellow) and nitrogen (red) atoms. The catalyst reduces oxygen to water (Image: Guosong Hong/Stanford University)